This invention relates to an inhalation device suitable for measuring the inspiratory strength of a patient and a method of using the same.
A number of inspiratory flow meters are known which may be used to measure inspiration flow rate, and length of inspiration achieved by a patient.
U.S. Pat. No. 5,167,506 describes an inhalation device training system which comprises a sensor which continuously measures the inhalation flow rate during inhalation through the mouthpiece and provides an electric signal which varies continuously with flow rate. The electrical signal is useful for monitoring inspiration flow rate, and length of inspiration, amongst other parameters.
Inhalation devices are known to be used for local administration of drugs to the respiratory tract and lungs of patients suffering from respiratory disorders e.g. asthma. Medicament for administration by inhalation should be of a controlled particle size in order to achieve maximum penetration into the lungs, preferably in the range of 1 to 10 micrometers in diameter and more preferably 1 to 5 micrometers. Unfortunately, powders in this particle size range, for example micronised powders, usually have very poor flow characteristics due to the cohesive forces between the individual particles which make them readily agglomerate together to form bridges which are not readily broken apart to become free flowing. These agglomerates of powder can be easily handled and may be used to fill powder inhalation devices. However, for efficient delivery to the lungs, the powder agglomerates must be broken down before they leave the device, back into a controlled size. It has been found that it is possible to break up powder agglomerates in the airflow as a user inhales by incorporating a series of baffles in the mouthpiece of a powder inhalation device. The baffles create turbulence and cause the air flow to collide with the baffles promoting the disintegration of powder agglomerates entrained in the air flow to render the powder in a form suitable for inhalation therapy. The use of these baffles in such devices creates a resistance to airflow within the device. Examples of such devices are Rotohaler(trademark), Diskhaler(trademark) and Turbohaler(trademark).
Different metered dose inhalers have different levels of resistance associated with them, because of the varying device designs. Persson""s study (Eur Respir J. 1997 10 681-684) has shown that the inhalation effort required to use inhalation devices effectively depends in the peak inspiratory flow achievable by the subjects. It is also acknowledged in this study that a minimum flow rate is needed to achieve efficient use of inhalation devices, and this flow rate can differ with type of device.
However, the problem with the inspiratory flow rate meters known is that whilst they measure the maximum inspiratory flow rate achievable by a patient under little or no resistance, they do not indicate the effort expended by a patient to achieve such a rate. The maximum flow rate achievable by a patient is affected by the resistance to airflow associated with a meter. In the context of inhalation therapy, more severely affected patients may not have the inspiratory strength to be able to achieve flow rates above a certain minimum, if the resistance of the meter is too great. This is important as the effectiveness for a given inhalation device is dependent on patient""s flow rate being above a certain minimum level, such levels differing between devices. It is therefore crucial to the selection of appropriate therapeutic treatment to be able to measure the inspiratory strength of a patient.
According to the invention there is provided a device having inspiratory resistance suitable for measuring the inspiratory strength of a patient which comprises a chamber in communication with a mouthpiece, said chamber being provided with a calibrated system for measuring the inhalation flow rate during inhalation through the mouthpiece and said chamber having a system by which the resistance to airflow through the device can be altered.
A device according to the invention is simple to use and provides clinicians with a useful tool with which to teach patients how to use various inhalation devices efficiently by allowing a patient to experience how to achieve the desired flow rate under the correct resistance. It also allows clinicians to select the most suitable inhalation device for a patient, taking into account the patient""s ability to produce the minimum airflow rate needed to operate the device efficiently.
Preferably the device comprises an elongate chamber defining a through-going pathway having first and second ends; wherein the calibration system comprises a sliding member which is retained within the chamber between the first and second ends and slides within the chamber when the pressure at the first end of the chamber is reduced by patient inspiration and where the variable resistance feature comprises means for varying the area through which the air enters the second end of the chamber such that the inspiratory resistance of the device may be varied by altering the area through which the air enters the chamber at the second end of the chamber.
Alternately and more preferably the device comprises an elongate chamber defining a through-going pathway having first and second ends; wherein the calibration system comprises a sliding member which is retained within the chamber between the first and second ends and slides within the chamber when the pressure at the first end of the chamber is reduced by patient inspiration and where the variable resistance feature comprises means for varying the area through which the air leaves the first end of the chamber to the mouthpiece such that the inspiratory resistance of the device may be varied by altering the area through which the air leaves the chamber at the first end of the chamber.
The chamber may at its first end incorporate a cap located between the first end of the chamber and the mouthpiece to reduce the area of the chamber.
Preferably the chamber is of circular cross section and more preferably tapered so that the cross section at the mouthpiece end is greater than at the second end.
Preferably the calibration system comprises calibrations along the length of the chamber such that the distance of travel of the sliding member along the chamber from its resting position during patient inspiration is indicative of the maximum inspiratory flow rate of the patient. In this embodiment the chamber will be transparent or translucent to enable the position of the sliding member to be seen.
Alternatively the calibration system comprises calibrations along the length of the sliding member such that the distance of travel of the sliding member along the chamber from its resting position during patient inspiration is indicative of the maximum inspiratory flow rate of the patient.
Preferably the sliding member is provided with means to guide its movement along the length of the chamber.
Even more preferably the guiding means comprise a rod located along the axis of the chamber along which the sliding member moves on patient inspiration.
Preferably a means for enabling the sliding member to be restored to its resting position after use is provided.
Even more preferably the means for enabling the sliding member to be restored to its resting position after use comprises a weight.
Preferably the means for varying the area through which the air enters the second end of the chamber or leaves the first end of the chamber comprises an iris, and even more preferably the iris has two or more defined aperture size settings.
Alternatively and more preferably the means for varying the area through which the air enters the second end of the chamber or leaves the first end of the chamber comprises the provision of two co-operating aperture bearing plates, such that by choice of relative orientation of the two plates the size of the area through which air enters the second end of the chamber or leaves the first end of the chamber, may be varied.
Preferably each plate is provided with at least two apertures.
Preferably two or more specified orientations of the two co-operating aperture-bearing plates are provided to define two or more specified areas through which air enters the device.
Even more preferably at least one specified orientation of the two co-operating aperture-bearing plates provides the device with a resistance matching that of an inhalation device.
In an alternative embodiment the area through which the air enters the second end of the chamber or leaves the first end of the chamber is defined by a ring formed between the inner surface of a portion of the chamber and the outer surface of a portion of a co-operating section having one or more apertures, at least one of the chamber and the co-operating section being tapered in said portion, whereby movement of the chamber relative to the co-operating section, along the longitudinal axis of the chamber alters the area of the ring, and hence the resistance to air flow. On inhalation by the patient, air may flow through the device, via the apertures and the ring.
Preferably the co-operating section is situated between the first end of the chamber and the mouthpiece.
Preferably the outer surface of the portion of the co-operating section is tapered.
It is particularly preferred that the taper is a curved taper.
More preferably both the inner surface of the portion of the chamber and the outer surface of the portion of the co-operating section are tapered.
Most preferably both tapers are curved tapers.
Preferably the curve of the taper will be configured to give resolution of the calibration scale.
Preferably the movement is rotational movement of the co-operating section relative to the chamber which leads to transitional movement of the co-operating section relative to the chamber along the longitudinal axis of the chamber. This facilitates calibration.
Preferably the co-operation between the chamber and co-operating section is by means of a screw and thread system.
Preferably two or more specified positions along the longitudinal axis of the device are provided to define two or more areas of the ring. Preferably these positions are defined by lining up a mark on the co-operating section with a mark on the chamber.
Even more preferably at least one specified position along the longitudinal axis provides the device with a resistance matching that of an inhalation device.
Preferably the co-operating section has at least two apertures.
Preferably the apertures are arranged symmetrically around the co-operating section.
A further aspect of the invention is a method of measuring the inspiratory strength of a patient by use of a device according to any preceding claim which comprises measuring the effort required to obtain a inspiratory flow rate as measured by the calibration system, at different resistance settings of the variable resistance feature.
Even more preferably this method comprises measuring the effort required to move the sliding member a desired distance along the chamber at different settings of the area through which the air enters the chamber at the second end of the chamber or leaves the first end of the chamber.
Another aspect of the invention is a device adapted for use with any conventional inspiratory flow rate meter which comprises a variable resistance feature providing means for varying the resistance to airflow through the inspiratory flow rate meter.